1. Field of the Invention
The present invention relates to an LC oscillator capable of reducing variations in the oscillation frequency.
2. Description of Related Art
The oscillation frequency of an LC oscillator is determined by its inductance and capacitance. Thus, the LC oscillator, which is used for a frequency synthesizer or the like, varies its oscillation frequency by varying its capacitance with its inductance fixed. As an LC oscillator that varies its oscillation frequency by digital control, a CMOS differential LC oscillator is known.
FIG. 17 is a circuit diagram showing a configuration of a conventional CMOS differential LC oscillator. In this figure, reference numerals 1 and 2 each designate an inductor, 3 and 4 each designate a variable capacitor, 5 and 6 each designate an NMOS transistor, and the reference numeral 7 designates a constant current source. The NMOS transistors 5 and 6 are connected in a cross-coupled state, and the variable capacitors 3 and 4 each consist of first to Kth capacitors connected in parallel, where K is an integer greater than one. The variable capacitors 3 and 4 are each controlled by a K-bit digital control signal TUNE. Specifically, the kth capacitor has its capacitance controlled by the kth bit of the digital control signal TUNE to vary the capacitance CT of the variable capacitors 3 and 4.
Thus, the oscillation frequency of the CMOS differential LC oscillator is varied by controlling the capacitance of the variable capacitors 3 and 4 by the digital control signal TUNE. It is designed such that when the capacitance CT1-1 of the first capacitor is adjusted to 1 by the digital control signal TUNE, the capacitance CTK-1 of the Kth capacitor becomes 2(Kxe2x88x921), where CTk-1 represents the capacitance of the kth capacitor. As a result, when controlled by the digital control signal TUNE, the CMOS differential LC oscillator shown in FIG. 17 varies its oscillation frequency discontinuously at every 2(Kxe2x88x921) step. Thus, to continuously vary the capacitance between the steps, an additional analog capacitor (not shown in FIG. 17) is used. Such an additional analog capacitor is connected in parallel with each of the variable capacitors 3 and 4 to be controlled by a control voltage fed from a charge pump circuit, for example. Thus, the analog capacitors vary the oscillation frequency, synchronize the phase and carry out tracking operation.
FIG. 18 is a diagram illustrating relationships between the digital control signal TUNE (TUNE code) and the oscillation frequency. In this figure, as the TUNE code varies from a minimum code (min code) to a maximum code (max code), the oscillation frequency varies continuously from the minimum frequency (fmin) to the maximum frequency (fmax).
The conventional LC oscillator with the foregoing configuration has the following problem. Assume that the line designated by the reference numeral 8 in FIG. 18 represents a desired frequency characteristic of the LC oscillator. In other words, it is designed to meet the characteristic 8. It is unavoidable, however, that a discrepancy occurs between the desired and designed values in the manufacturing process of individual CMOS differential LC oscillators. Thus, it is not unlikely that the CMOS differential LC oscillator cannot satisfy the desired frequency characteristic.
In addition, it is unavoidable that the CMOS differential LC oscillator has variations due to the ambient temperature in its use environment. As a result, as indicated by the reference numeral 10 or 11 in FIG. 18, the frequency characteristic of the CMOS differential LC oscillator can deviated from the desired frequency characteristic, thereby bringing about variations in the frequency characteristic. In this case, the conventional CMOS differential LC oscillator cannot eliminate the variations in the frequency characteristic, presenting a problem of reducing the yield of the CMOS differential LC oscillator.
The present invention is implemented to solve the foregoing problem. It is therefore an object of the present invention to provide an LC oscillator capable of increasing the yield by correcting the deviation of the frequency characteristic.
According to a first aspect of the present invention, there is provided an LC oscillator including variable capacitor means for varying its capacitance in response to a control voltage supplied in accordance with a digital control signal; additional variable capacitor means for varying its capacitance in response to an additional control voltage; and adjusting means for adjusting the oscillation frequency to one of the maximum oscillation frequency and minimum oscillation frequency by varying the capacitance of the variable capacitor means by the control voltage with fixing the digital control signal, wherein the additional variable capacitor means adjusts the oscillation frequency to the other of the maximum oscillation frequency and minimum oscillation frequency by varying its capacitance by the additional control voltage with fixing the digital control signal.
According to a second aspect of the present invention, there is provided an LC oscillator including variable capacitor means for varying its capacitance in response to a control voltage supplied in accordance with a digital control signal; and adjusting means for adjusting the oscillation frequency to the maximum oscillation frequency and the minimum oscillation frequency by varying a first voltage and a second voltage with fixing the digital control signal, the first voltage and second voltage being selectively supplied to the adjusting means in response to the digital control signal, and the first voltage being higher than the second voltage.
The foregoing configurations can correct the deviation in the frequency characteristic and set the oscillation frequency at a desired oscillation frequency, thereby offering an advantage of being able to improve the yield.